At present, the principle of detecting a gas concentration is generally based on non-dispersive infrared (NDIR) technique, i.e. an infrared light with a specific waveband is selected and allowed to pass through a sample gas according to the absorption characteristics of the sample gas to this infrared light, the relationship between the attenuated intensity of the infrared light and the concentration of the sample gas to be detected follows the Beer-Lambert law. For instance, as to an apparatus for detecting concentration of CO2 in the breathing gas, since an obvious absorption peak of CO2 molecule occurs as the wavelength of the infrared light is 4.26 micrometer, the concentration of CO2 gas can be calculated in terms of the attenuated intensity of the infrared light with the wavelength of about 4.26 micrometer. In order to compute the concentration of CO2 gas, CO2 gas with known concentration is input into the apparatus, and the corresponding intensity of the infrared light is detected and recorded so that an intensity-concentration curve can be obtained. Later, the concentration of CO2 contained in a gas to be detected can be determined by means of the reference intensity-concentration curve according to the detected intensity of infrared light. Owing to the differences in a light resource, a detector (i.e. sensor), a circuit for each detecting apparatus, the corresponding calibrating factor must be preset before shipping out of the factory. By inputting CO2 gas with known standard concentration into the apparatus and comparing the difference between the detected concentration and the standard concentration, the calibrating factor is determined and used for correcting the measured results of the concentration. However, the detected results by the apparatus may be not accurate due to the ageing problem of the light source, the detector, etc. after long time service. Therefore, the periodical maintenance is required every half year or one year. That is, the calibration process should be implemented again by inputting CO2 gas having a standard concentration. This calibration process is complex and inconvenient because some equipment, such as a gas cylinder filled with CO2 gas having a standard concentration, gas pressure regulator, pipes and so on, need to be prepared.
In order to maintain the accuracy of the detected results during its long period of service time, dual beam mode is usually employed. With two light paths being generated by a beam splitter in the apparatus, one light path passes through the gas to be detected, and the other one as a reference light path passes through a sealed reference gas. The concentration of the sealed reference gas in the reference light path is known and thereby used as a reference concentration, so that automatic calibration process can be realized, for example in U.S. Pat. Nos. 5,764,354 and 5,077,469. However, in the detecting apparatus with dual beam mode, the intensities of two light beams are measured by two detectors respectively. Since the aging extents of two detectors are different after long time service and the difference between two detectors themselves always exist, the error in the detected results may exist during the detection process.
Therefore, those apparatuses for detecting the concentration of CO2 with the absorption characteristics of infrared light in the prior art have the following disadvantages:
1. Because the apparatus need to be calibrated periodically and calibration process is complex, cost of maintenance for the apparatus is high.
2. For dual beam mode, two detectors are used to detect light intensities in two light paths. But, since the aging extents of two detectors are different after long time service and the difference between two detectors themselves always exist, the accuracy of the detected results may be deteriorated.